docs: def _extract_psf_fitting_names(psf):
      """
      Determine the names of the x coordinate, y coordinate, and flux from
      a model.  Returns (xname, yname, fluxname)
      """
  
      if hasattr(psf, 'xname'):
          xname = psf.xname
      elif 'x_0' in psf.param_names:
          xname = 'x_0'
      else:
          raise ValueError('Could not determine x coordinate name for '
                           'psf_photometry.')
  
      if hasattr(psf, 'yname'):
          yname = psf.yname
      elif 'y_0' in psf.param_names:
          yname = 'y_0'
      else:
          raise ValueError('Could not determine y coordinate name for '
                           'psf_photometry.')
  
      if hasattr(psf, 'fluxname'):
          fluxname = psf.fluxname
      elif 'flux' in psf.param_names:
          fluxname = 'flux'
      else:
          raise ValueError('Could not determine flux name for psf_photometry.')
  
      return xname, yname, fluxname